This disclosure relates to testing and evaluating cathodic protection effectiveness on buried or submerged metallic structures, and specifically to the evaluation of corrosion protection levels on pipelines, tanks, piles, and piping systems.
Buried or submerged metallic structures, such as pipelines, tanks, and distribution piping systems are usually coated with non-conductive material to prevent corrosion. If any corrosion occurs in any uncoated areas of the structure adverse effects may occur which will reduce the effective life of the structure. To prevent such adverse effects, most pipelines are provided with corrosion protection comprising cathodic protection, in addition to the non-conductive coating. Cathodic protection provides corrosion protection to any bare metal areas exposed to soil due to coating defects, by causing direct current to flow from the soil into the structure, thereby polarizing the structure as a cathode. The required direct current output of the cathodic protection system is reduced to manageable levels by the coating, which substantially reduces the bare metal area of the structure exposed to soil.
The objective of the cathodic protection is to shift the potential of the structure to a more negative potential. The potential shift must be large enough to mitigate structure corrosion. Potential criteria have been developed by the National Association of Corrosion Engineers (RP0169-92) to provide guidance for determination of safe cathodic protection levels to mitigate corrosion. One of the criteria is based on a single value of potential, measured with a regular high-impedance voltmeter with the cathodic protection system operating. The potential measured with the cathodic protection system operating are identified as “on” potential readings. This measurement is very easy to take, however, it requires a consideration or elimination of voltage drops in resistive materials between the reference electrode and the structure. Another criterion is based on achieving the same value of structure potential immediately after interrupting the operation of the cathodic protection system, and is referred to as an “off” potential reading. A further criterion is based on a single value of the structure potential decay, which is measured from the “off” potential, leaving the cathodic protection system disconnected for several hours or days.
There is no easy and practical method to determine the voltage drop when the “on” potential reading is taken. Therefore, the “off” potential readings, which eliminate the soil voltage drop measured immediately after interrupting the cathodic protection system from the structure, are often used for monitoring corrosion protection levels. However, the “off” potential readings are much more difficult to take than the “on” readings. The interpretation of the “off” potential readings is also much more complex. The “off” potential readings often require use of synchronized current interrupters, fast reacting recorders, oscilloscopes, or wave analyzers. The “off” potential readings after cathodic protection is interrupted can be adversely affected by long-cell currents in the structure caused by currents flowing between more polarized sections of the structure being protected, which occur in the proximity of the rectifiers, and less polarized sections of the structure being protected, which typically occur at sections of the structure that are generally equidistant between sequential rectifiers. Also, the “off” potential readings are often adversely affected by inductive or capacitive voltage spikes, caused by cathodic protection interruption. If the “off” potential reading is taken some time after the spike, some of the polarization is lost and the reading could be therefore invalid.
Meeting the “off” potential criterion often requires that more cathodic protection current be applied than is required to meet the “on” potential criterion, resulting in possible overprotecting of the structure, faster deterioration of the coating, and a higher probability of hydrogen evolution and steel embrittlement within the structure. The “off” potential measurements are not valid in areas where substantial uninterruptable direct currents are flowing through the soil into or from the structure, polarizing the structure. Such conditions exist, for instance in stray current areas, where the structure is affected by stray currents from electric railroads, from cathodic protection systems on foreign structures, and in areas with telluric (earth) currents naturally induced by fluctuations in the earth's magnetic field. Also, the “off” potential readings cannot be used on structures with distributed galvanic anodes directly connected to the structure.
To eliminate some of the disadvantages of the “off” potential readings on the structure, different cathodic protection test probes and coupon/access tube assemblies have been proposed. The probes consist of a short steel pipe section as a coupon, a plastic tube filled with conductive backfill functioning as an electrolytic “salt bridge,” and a porous ceramic plug glued to the end of the plastic tube, representing a potential sensing area. A coupon is a metal electrode, which simulates an area in which the non-conductive coating is not present on the structure and provides a reference from which the cathodic protection system can be measured. Coupons are made from the same or similar metal as the structure, and are electrically connected to the structure to receive cathodic protection. Cathodic protection probes with cylindrical coupons, now commercially available, have been described in Material Performance, published by National Association of Corrosion Engineers, Houston, Tex., June 1996, pp. 21–24.
Cathodic protection probes with cylindrical coupons are difficult to use when the structure to be tested is located beneath a surface obstruction that prevents access to the soil from the ground surface without interactions with the surface obstruction. Conducting periodic inspections of structures located beneath surface obstructions, such as paved surfaces, using currently available methods is costly and destructive.